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Electrochemical treatment of simulated sugar industrial effluent: Optimization and modeling using a response surface methodology
⁎Corresponding author. Tel.: +914312503120. math_chem95@rediffmail.com (Manickam Matheswaran)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The removal of organic compounds from a simulated sugar industrial effluent was investigated through the electrochemical oxidation technique. Effect of various experimental parameters such as current density, concentration of electrolyte and flow rate in a batch electrochemical reactor was studied on the percentage of COD removal and power consumption. The electrochemical reactor performance was analyzed based on with and without recirculation of the effluent having constant inter-electrodes distance. It was found out that the percentage removal of COD increased with the increase of electrolyte concentration and current density. The maximum percentage removal of COD was achieved at 80.74% at a current density of 5 A/dm2 and 5 g/L of electrolyte concentration in the batch electrochemical reactor. The recirculation electrochemical reactor system parameters like current density, concentration of COD and flow rate were optimized using response surface methodology, while COD removal percents were maximized and power consumption minimized. It has been observed from the present analysis that the predicted values are in good agreement with the experimental data with a correlation coefficient of 0.9888.
Keywords
Electrochemical oxidation
Sugar industrial effluent
Electrolyte
Current density
Response surface methodology
1 Introduction
The industrialization and modification of manufacturing processes have resulted in an increase in the volume of wastewater discharge into the environment which causes water pollution (Manisankar et al., 2003). India is one of the largest producers and consumers of 22 million tons of sugar per annum in the world. The sugar industries utilized huge quantities of water in the results to produce large amounts of wastewater. In the industry, around 1500–2000 L of water is used and generates about 1000 L of wastewater per ton of cane crushing. The effluent mainly comes from floor washing and condensate water, leakage and spillages of sugarcane juice from valves and glands of the pipeline, syrup and molasses in a different section, solid waste etc. The environmental issues are disposal of effluent containing molasses, wastewater, solid waste and by-products into the ecosystem and land after treatment of wastewater in the sugar manufacturing process.
The wastewater produced in the sugar manufacturing process has a high content of organic material and subsequently high Biochemical Oxygen Demand (BOD), particularly because of the presence of sugars and organic material in the beet or cane. In cane processing, the typical levels of BOD are 1700–6600 ppm in the untreated effluent , the Chemical Oxygen Demand (COD) is from 2300 to 8000 ppm and the total suspended solids are up to 5000 mg/L, and the ammonium content is high (WORLD BANK GROUP, 1998). The wastewater may also contain pathogens from contaminated materials or production processes. The effluent often generates odor and dust, which need to be controlled. Many processes have been developed to treat this effluent such as electrochemical oxidation (Guven et al., 2009; Mancera et al., 2010), biosorption (Martín-Lara et al., 2010), membranes separation (Hinkova et al., 2002), and biochemical oxidation (Prasad et al., 2006). Only a limited number of researches have been carried out using electrochemical oxidation for the treatment of the sugar industrial effluent.
Electrochemical treatment method may be considered as an economically alternate process under such conditions when conventional treatment fails to reduce pollution (Guven et al., 2008). The electrochemical oxidation is one of the advanced oxidation processes, potentially a powerful method of pollution control, offering high removal efficiencies in compact reactors with simple equipments for control and operation. These processes generally operate at a low temperature and usually prefer adding electrolyte solutes to increase the conductivity of wastewater. In recent years, there has been a growing interest in the treatment of industrial effluents by electrochemical methods. Many researchers had investigated the electrochemical oxidation for the treatment of various types of wastewater containing phenol (Maa et al., 2009), pentachlorophenol (Upendra et al., 2008), tannery (Rao et al., 2001), olive mill (Canizares et al., 2006), coffee curing (Bejankiwar et al., 2003), and textile wastewater (Radha et al., 2009), resin (Prabhakaran et al., 2009), pharmaceutical effluent (Abhijit et al., 2005), deproteinated whey wastewater (Guven et al., 2008), distillery spent wash (Krishna Prasad and Srivastava, 2009), dairy manure (Ihara et al., 2006), and organic pollutants (Martinez-Huitle and Ferro, 2006). However, there is no study carried out for the treatment of synthetic sugar effluents by electrochemical oxidation methods.
In addition the electrochemical parameters of the study were optimized statistically by adopting Response Surface Methodology (RSM). RSM is a designed regression analysis to predict the value of a dependent variable based on the controlled values of the independent variables. It leads to the need for an experimental design, which can generate a lot of samples for consumer evaluation in a short period of time, and thus laboratory level tests are more efficient. From the parameter estimates, it can be determined which variable contributes the most to the prediction model, thereby allowing the product researcher to focus on the variables that are most important to the product acceptance. RSM was used to optimize the experimental parameter for a different process, which includes an advanced oxidation process (Otto, 1999), electrochemical oxidation (Gursesa et al., 2002) and adsorption (Montgomery, 2002). The two most common designs used in RSM are the Central Composite Design (CCD) and the Box–Behnken Design (BBD). In the present study, BBD with RSM was adopted to optimize the experimental parameters like various operation parameters such as current density, flow rate and effluent concentration on the COD removal efficiency and power consumption.
The main objective of the present study is to assess the electrochemical oxidation treatment of synthetic sugar effluents using RuO2 coated titanium as an anode and stainless steel as a cathode. Experiments were conducted in a batch electrochemical reactor with and without recirculation to investigate the effect of operating parameters such as current density, flow rate and concentration of effluent, and effluent on the percentage removal of COD and energy consumption. An attempt has been made to employ BBD using RSM for optimizing the key influencing parameters (i.e. current density, flow rate and effluent concentration) on removal of COD and power consumption in a batch recirculation system.
2 Materials and methods
2.1 Chemical reagents
All the chemicals used in the study were of analytical reagent grade. NaCl (Merck) in high purity was used as a supporting electrolyte. RuO2 coated on titanium electrodes were obtained from Titanium Tantalum Components Industries, India. Stainless Steel (SS304) electrodes were manufactured in Carbone Lorraine India Pvt. Ltd., India. The simulated sugar effluent prepared by dissolving an appropriate amount of saccharose (1 M) and 0.5 M ammonium carbonate, potassium dihydrogen phosphate and calcium hydroxide in distilled water was used for experimental studies. The wastewater was buffered with sodium bicarbonate to prevent a drop in pH.
2.2 Experimental method
Experimental setup of the batch electrochemical reactor was made up of a cylindrical glass container, closed with a lid which helps fit the electrodes in a position to maintain the inter-electrode distance. An RuO2 coated titanium expanded mesh served as an anode and stainless steel as a cathode having electrode dimensions of 10 × 8 cm. The electrodes were connected with regulated power supply (L1606, Aplab Limited) to supply electrical energy to the system. The lid was designed to facilitate the sample collection and the stirring was done with the magnitude stirrer. The synthetic sugar effluent was taken in the electrochemical reactor with a volume of 1000 ml. The experiment was conducted under galvanostatic conditions fixing the reservoir volume in the batch electrochemical reactor. During the electrochemical reaction, the organic compound in the effluent was oxidized at the anode and a reduction reaction occurred at the cathode.
The effect of experimental parameters like current density was varied from 1 to 5 A/dm2, mediator concentrations from 1 to 9 g/L was studied. During the process, cell voltage was noted down periodically. The samples were collected at various intervals of time for the analysis of organic concentration. The concentration of organics was measured in terms of COD. The COD value was determined by dichromate closed reflux method (APHA, 1995; Balaji et al., 2007).
Similar experiments were also performed in the batch recirculation electrochemical reactor. The electrochemical reactor setup was a filter press type model with an RuO2 coated titanium expanded mesh as the anode and stainless steel as the cathode. The fixed volume of effluent was continuously recirculated between the reactor and the reservoir using a centrifugal pump at a constant flow rate. The volume of the reactor is 0.098 dm3. The effluent was treated under the galvanostatic mode. The effects of various experimental parameters like current density and feed flow rate on percentage COD removal were studied.
The COD of the samples was determined by the standard method. The percentage of COD removal and power consumption was calculated by using the following formulae:
In the present study, BBD was applied to investigate and validate the process parameters affecting the removal of COD and power consumption by batch recirculation electrochemical oxidation. Current density (X1), feed flow (X2) and effluent concentration (X3) are input variable parameters. The interval of the allowed values for these factors was deduced from the preliminary tests carried out (Table 1). The factor levels were coded as −1 (low), 0 (central point or middle) and 1 (high). For this response (Y), a polynomial model of the second degree is established to quantify the influence of the variables.
Factor
Original factor (X)
Coded factor (X)
−1
0
+1
Flow rate (L/h)
X1
20
60
100
Current density (A/dm2)
X2
1
3
5
Effluent concentration (ppm)
X3
500
1500
2500
3 Results and discussion
3.1 Batch without recirculation operation
3.1.1 Effect of current density
An important operating variable of the electrochemical process is the current density, which is the current input divided by the surface area of the electrode. The experiments were carried out at different current densities 1, 2, 3, 4 and 5 A/dm2 at fixed concentrations of the electrolyte and effluent. Fig. 1 shows that the percentage removal of COD from 44 to 80% and power consumption from 0.98 to 10.2 kWhr/kg COD increases with the increase in current density. At the same time, the performance of the reactor will be affected under different current densities while altering the other operating conditions simultaneously because the generation of chlorine/hypochlorite depends on mass and charge.
Effect of current density in the batch electrochemical reactor without recirculation on percentage COD removal and power consumption.
3.1.2 Effect of electrolyte concentration
The effect of electrolyte concentration during the electrochemical oxidation of organic compounds was studied on the removal of COD and power consumption under the constant current density and effluent concentrations as shown in Fig. 2. It was observed from the figure, that the percentage of COD removal increased from 38 to 80.74% with the increasing concentration of NaCl from 1 to 9 g/L, but the power consumption was decreased from 19 to 10.77 kWhr/kg COD. The presence of NaCl in the reaction medium generates in situ very strong oxidants of HOCl/ClO− increase in the electrolyte concentration and the voltage directly decreases the concentration of these chemicals in the medium enabling faster COD removal. The variation of cell voltage, anode and cathode potentials may be due to the influence of the kinetic characteristics of the reaction, which is not considered by us. It can also be observed from Fig. 2 that the supporting electrolyte plays a major role in the degradation of the organic matter present in wastewater.
Effect of electrolyte concentration in the batch electrochemical reactor without recirculation on percentage COD removal and power consumption.
3.2 Batch recirculation operation
3.2.1 Effect of current density
Experiments were carried out at different current densities from 1 to 5 A/dm2 with an interval of 1 A/dm2 keeping the other parameters constant. The effect of current density on COD removal and power consumption is presented in Fig. 3 and it shows that the COD removal and power consumption increases with increasing current densities. The cell voltage increases gradually with the increase in current densities as can be expected. There is a slight increase in the temperature with the increase in current densities because of poor conductivity of the solution. Hence an external cooling system is needed to maintain constant temperature.![Effect of current density in the batch reactor with recirculation on percentage COD removal and power consumption ([COD] = 500 ppm, flow rate = 60 L/h, [NaCl] = 4 g/L).](/content/184/2016/9/2_suppl/img/10.1016_j.arabjc.2011.10.004-fig3.png)
Effect of current density in the batch reactor with recirculation on percentage COD removal and power consumption ([COD] = 500 ppm, flow rate = 60 L/h, [NaCl] = 4 g/L).
3.2.2 Effect of flow rate
Experiments were conducted at five flow rates, keeping other parameters constant. It can be ascertained from Fig. 4 that the COD removal percentage decreases and power consumption increases with increasing flow rate. At a current density of 5 A/dm2, the maximum percentage removal of COD 83.94% is observed and energy consumption is 6.64 kWhr/kg COD at a flow rate of 20 L/h.![Effect of flow rate in the batch reactor with recirculation on percentage COD removal and power consumption ([COD] = 500 ppm, current density = 3 A/dm2, [NaCl] = 4 g/L).](/content/184/2016/9/2_suppl/img/10.1016_j.arabjc.2011.10.004-fig4.png)
Effect of flow rate in the batch reactor with recirculation on percentage COD removal and power consumption ([COD] = 500 ppm, current density = 3 A/dm2, [NaCl] = 4 g/L).
The percentage COD removal in the recirculation operation was higher compared to the one without recirculation. This may be due to increase in the mass transfer rate of the recirculation operation.
3.3 Statistical analysis and modeling
The most important factors that affect the electrochemical oxidation process are current density, flow rate and effluent concentration of the effluent in the batch recirculation reactor. In order to study the combined effect of these factors the experiments were conducted at different combinations of operating parameters. The encoded values and the corresponding percentage of COD removal and power consumption along with predicated values are given in Table 2.
Standard order
Flow rate (L/h)
Current density (A/dm2)
Effluent concentration (ppm)
% COD removal
Power consumption kWhr/kg of COD
Actual
Predicted
Actual
Predicted
1
20
1
1500
27.69
29.62
1.27
1.40
2
100
1
1500
29.85
27.33
3.33
2.75
3
20
5
1500
55.89
58.41
6.64
8.23
4
100
5
1500
52.16
50.23
18.77
21.47
5
20
3
500
65.35
63.33
7.52
8.10
6
100
3
500
56.27
58.70
20.34
19.80
7
20
3
2500
35.42
32.99
3.01
3.55
8
100
3
2500
25.13
27.15
7.02
6.44
9
60
1
500
40.62
40.71
4.14
5.26
10
60
5
500
80.15
79.65
28.34
24.85
11
60
1
2500
22.36
22.86
1.65
3.11
12
60
5
2500
35.69
35.60
10.21
9.09
13
60
3
1500
48.21
48.21
7.11
7.11
14
60
3
1500
48.21
48.21
7.11
7.11
15
60
3
1500
48.21
48.21
7.11
7.11
16
60
3
1500
48.21
48.21
7.11
7.11
17
60
3
1500
48.21
48.21
7.11
7.11
The regression method was used to fit the second order polynomial to the experimental data and to identify the relevant model term. The final equations obtained in terms of uncoded factors for percentage COD removal and power consumption are given by the Eqs. (3) and (4), respectively.
The statistical significance of the ratio of mean square variation due to regression and mean square residual error was tested using ANOVA. The ANOVA for the second order equation fitted for COD removal and power consumption is presented in Tables 3 and 4, respectively. The ANOVA results for COD removal and power consumption by the electrochemical oxidation system (F value) were 68.74 and 32.12, respectively. The large value of F indicates that most of the variation in the response can be explained by the regression equation. The associated p value is used to estimate whether F is large enough to indicate statistical significance. Any factor or interaction of factors with p < 0.05 is considered to be significant. The probability p (∼0.0001) is less than 0.05. This indicates that the model is statistically significant. The ANOVA indicated that the equation adequately represented the relationship between the response (the percentage COD and power consumption) and a significant variable. The model gave the coefficient of determination (R2) values of 0.9888 and 0.9764 adjusted R2 values of 0.9744 and 0.9459 for COD removal and power consumption, respectively.
Source
Sum of squares
df
Mean square
F value
P-value prob > F
Remarks
Model
3591.18
9
399.02
68.74
<0.0001
Significant
X1
54.81
1
54.81
9.44
0.0180
Significant
X2
1335.67
1
1335.67
230.09
<0.0001
Significant
X3
1915.50
1
1915.50
329.97
<0.0001
Significant
X1X2
8.67
1
8.67
1.49
0.2611
–
X1X3
0.37
1
0.37
0.06
0.8089
–
X2X3
171.61
1
171.61
29.56
0.0010
Significant
37.58
1
37.58
6.47
0.0384
Significant
61.60
1
61.60
10.61
0.0139
Significant
0.43
1
0.43
0.07
0.7931
–
Source
Sum of squares
df
Mean square
F value
P-value prob > F
Remarks
Model
723.72
9
80.41
43.41
<0.0001
Significant
X1
106.51
1
106.51
57.50
0.0001
Significant
X2
326.53
1
326.53
176.28
<0.0001
Significant
X3
160.38
1
160.38
86.59
<0.0001
Significant
X1X2
35.40
1
35.40
19.11
0.0033
Significant
X1X3
19.40
1
19.40
10.48
0.0143
Significant
X2X3
46.31
1
46.31
25.00
0.0016
Significant
0.06
1
0.06
0.03
0.8587
–
6.34
1
6.34
3.43
0.1067
–
21.13
1
21.13
11.41
0.0118
Significant
3.4 Optimization of operating conditions
The response surface contour plots of COD removal efficiency and power consumption over independent variables such as current density, flow rate and effluent concentration are shown in Figs. 5 and 6. These graphical representations are derived from the models of Eqs. (3) and (4). The contour plots given in Fig. 5 show the relative effects of two variables when the volume of solution is kept constant at its zero level. It is to be noted that the removal efficiencies and power consumption increase with increasing current density. By increasing the recirculation flow rate, the COD removal efficiencies and power consumption do not affect much as shown in Fig. 6.
Three-dimensional response surface graphs for percentage COD removal and power consumption versus concentration and flow rate.
Three-dimensional response surface graphs for percentage COD removal and power consumption versus flow rate and current density.
4 Conclusion
The electrochemical oxidation process was used to treat the synthetic sugar industry effluent using both batch reactors with and without recirculation. The influences of operating parameters such as current density and electrolyte concentration on percentage COD removal and power consumption were investigated. For a maximum percentage removal of COD 83.94% the batch recirculation reactor requires only 6.64 kWhr/kg COD of energy whereas without recirculation requires 10.77 kWhr/kg COD for a removal efficiency of 80.74%. Hence it can be concluded that a batch reactor with recirculation is the better option when compared to a batch reactor without recirculation. RSM has been used to study the effects of various parameters on the removal of COD and power consumption for a batch reactor with recirculation and quadratic models have been generated. The comparison of experimental and predicted values by the model for each response shows that the error is less than 3%. Thus it is concluded that this technology can be used on a large scale for the treatment of real sugar industrial effluents.
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